PROGENITOR REGULATION UNDERLYING CORTICAL INTERNEURON SPECIFICATION Inhibitory GABAergic interneurons modulate complex cortical circuit operation. Most cortical interneurons originate from medial ganglionic eminence (MGE) progenitors that express the transcription factors, NKX2.1 in the ventral MGE and NKX6.2 in the dorsal MGE. While excitatory principal neuron production has been extensively studied, understanding of interneuron genesis, particularly the behavior and regulation of MGE progenitors, remains very limited. The proposed project addresses this outstanding knowledge gap. Although mature cortical interneurons are highly diverse, primary decisions to adopt a glial or somatostatin (SOM+) or parvalbumin (PV+) expressing neuronal fate are made as progenitors in the MGE. Based on strong preliminary data from the Shi and Ross laboratories, we hypothesize that a tight spatial and temporal regulation of the behavior and gene expression properties of MGE progenitors is essential for proper production of interneurons destined for the cortex. The project encompasses two major goals: Aim 1 probes the functional interactions between progenitor cell polarity determinant, partition defective 3 (PARD3), and G1-phase active cell cycle proteins, cyclins D1 (cD1) vs. D2 (cD2), in regulating MGE cell division, testing 2 hypotheses: 1a. MGE progenitor division modes and dynamics are spatially and temporally regulated to generate proper PV+ and SOM+ interneuron numbers. Retroviral and MADM technologies with computational modeling will be used to determine spatial (dorsal vs. ventral) and temporal dynamics of MGE radial glial progenitor (RGP) and intermediate progenitor cell (IPC) division modes and interneuron output. 1b. Differential interactions between PARD3 and cD1 vs. cD2 coordinate MGE progenitor division mode and dynamics to regulate proper PV+ vs. SOM+ fates. Studies pursue differential interactions between PARD3 and cD2 vs. cD1 in governing division mode, dynamics, and cortical interneuron output of MGE RGPs and IPCs. Aim 2 explores the core molecular program regulating the spatial and temporal behavior of MGE RGPs and IPCs using 10X Genomics-based single cell RNA sequencing of wildtype, cD2?/?, cD1?/?, Pard3 cKO and Pard3cKO;cD2?/? double mutants. Hypothesis: MGE output depends on gene expression defining cD1 and cD2 dependent progenitor pools, their division mode and dynamics. Expression networks will be validated by histological and molecular manipulation, including CRISPR/Cas9 genome editing of candidates for key drivers of MGE progenitor specification and whose expression is altered in cD1 or cD2 and Pard3 mutant MGE. The project will address a substantial knowledge gap regarding cortical interneuron genesis and provides a fundamental framework for the spatial and temporal regulation of MGE progenitors, elucidating their division mode and dynamics, their interneuron output, and the underlying core program coordinating MGE progenitor division regulation and interneuron specification.